Secret transmission system



3 Sheets-Sheet 1 Filed Sept. 11, 1942 Fig. 3

Sept. 3, 1946. M. M. LEVY 2,406,841

SECRET TRANSMISSION SYSTEM Filed Sept. 11, 1942 3 Sheets-Sheet 2 a V1 I-|l| 1|| -lll "Il 4, I E132 b ATTORNEY Sept. 3, 1946. M. M. LEVY.2,406,341

SECRET TRANSMISSION SYSTEM Filed Sept. 11, 1942 3 Sheets-Sheet 5 Hg, 9.H510.

may zmo 3000 4000 5000 I050 30 00 40 00 50 00 -FigiJZ.

AL fle l/VVE/VTOR a) dmjumw Patented Sept. 3, 1946 SECRET TRANSMISSIONSYSTEM Maurice Moise Levy, London W. C. 2, England, assignor to StandardTelephones and Cables Limited, London, England, a British companyApplication September 11, 1942, Serial No. 458,061 In Great Britain July9,1941

8 Claims.

This invention relates to an electric communication system in whichmessages are transmitted in such form as to be unintelligible to areceiver not possessing the requisite key, hereinafter called a secrecysystem.

The present invention consists in its broadest aspect in such a systemcomprising means for transmitting two sets of signals, one on each oftwo separate channels, the first set of signals comprising the desiredmessage in such form that the signals by themselves do not convey themessage intelligibly, and the second set being by itself unintelligible,the two sets of signals on being combined being capable of yielding thedesired message in intelligible form.

The invention is applicable to transmission both over wires and by meansof electromagnetic radiation. In the case of transmission, either overwires or by means of electromagnetic radiation, by means of a carrierwave, the invention need not involve any greater band width than thetransmission of one of the sets of signals alone. It is known that twoseparate sets of signals may be transmitted on a single carrier wave,one by frequency or phase modulation of the carrier wave and the otherby amplitude modulation of the carrier wave, and that both sets ofsignals, whilst occupying the same band width may be separately receivedby two different processes of demodulation. Accordingly in performingthe present invention the two sets of signals may be transmitted, theone by frequency or phase modulation and the other by amplitudemodulation of a Single carrier.

The invention is equivalent to transmitting an unintelligible set ofsignals on one channel and to transmitting on the other channel part ofthe key, in unintelligible form, needed to cause the first mentioned setof signals to render an intelligible message, the remainder of the keyconsisting in the knowledge of how to apply the second set of signals tothe first. This may not in all cases yield a very high degree of secrecybut may be made to do so by means of further features of the invention.This aspect of the matter may be made clearer by means of an example.

It is well known to mask speech by means of noise, this consisting of arandom series of frequencies continually varied in different ways, thereceiver being provided with a similar source of noise, and meanscontinually varied in synchronism with the means at the transmitter. Thedesired message is then obtained by subtracting the local noise N at thereceiver from the signals which consist of speech S or other desiredsignals plus noise N. In applying the invention to this form of securingsecrecy, the signals S-l-N are transmitted on one channel and the noiseN alone on another channel. The signals sent along either channel arethemselves unintelligible but the key needed to cause the signals S+N toyield an intelligible message is partly the signals N transmitted on theother channel and partly the knowledge that these signals N must besubtracted from the signals S+N. This by itself does not give a highdegree of secrecy since the knowledge that the desired signals may beobtained by subtracting the two sets of signals is of such a nature thatit can readily and simply be conveyed to an unauthorised person.

If, however, the noise currents before being mixed with desired signalsto constitute one of the transmitted sets of signals, are passed througha network which alters their amplitude and phase in a manner dependentupon frequency, the result is different from the original noise and maybe called N/(p. One set of signals now consists of S+N/1p and the otherof N. Both are unintelligible. The set of signals N still contains partof the key needed to obtain the desired signals from the set S+N/rp butthe desired signals S cannot be obtained by simple subtraction of thetwo sets of signals. The signals N must first be passed through anetwork at the receiver which is identical with the one used at thetransmitter and the result N/(P then subtracted from the set of signalsS+N/ The network may be of fairly simple form and yet the constitutionthereof cannot be determined from either set of signals picked up by anunauthorised listener or from any combination of the two. As there is avery large number of forms that the network might take, it is highlyunlikely that an unauthorised listener could find the correct form by aprocess of trial and error.

This process may be modified by using two networks, one at thetransmitter and one at the receiver which are the inverse, one of theother. Thus the desired signals S may be combined with noise N and theset of signals S+N transmitted on one channel. The noise N is alsopassed through a network of the kind mentioned above to yield a set ofsignals N/ g) which is transmitted on the other channel. Each set ofsignals is by itself unintelligible and the result of subtracting onefrom the other is equally unintelligible. If, however, the signals N/ pare at the receiver passed through a network which is the inverse ofthat at the transmitter so as to restore the set of signals N/ to N, thedesired signals are obtained in intelligible form by the subtraction ofthe set of signals N from the set of signals S+N.

The invention may also be applied in cases in which other means thanmixture with noise is used to render speech or other signalsunintelligible. Thus secrecy systems are known in which a carrierfrequency is continuously varied, but in known forms of such systemselaborate synchronising systems are needed to vary in synchronism thefrequencies of the carrier at the transmitter and the local oscillatorof a heterodyne modulator at the receiver. These difficulties areremoved by means of the present invention.

Examples of such use of the present invention will now be described. Itis well known that in verted speech may be obtained by causing thespeech band to modulate a carrier frequency near the upper limit of theband andby selecting the lower side band. Such inverted speech is byitself unintelligible, but it is easy to render it intelligible byrecombining it with the original modulating frequency, easily found bytrial if it is not known, and again selecting the lower side band withsuppression ofthe carrier. If, however, the carrier frequency used toproduce the-inverted speech be continuously varied in an irregularmanner, it is almost impossible to obtain intelligible speech therefromwithout having a local oscillator varied in synchronism with saidcarrier. In one example of the use of the present invention invertedspeech thus produced with a continuously varied carrier is transmittedon one channel, and a mixture of the said inverted speech and thecarrier used to produce it is transmitted on a second channel. At thereceiver, intelligible signals can be obtained by combining the signalsreceived on the two channels. Preferably, before combining the varyinginverted speech with the varying carrier to constitute the signals senton the second channel, the varying inverted speech is passed through adistorting network and a similar network is used at the receiver toobtain from the signals received on the second channel signals which maybe combined with those received onthe first channel to produceintelligible speech. 7 In the alternative the varying inverted speech ispassed through a distorting network before being transmitted on thefirst channel and the received signals passed through an inverse networkbefore being combined. with those received on the second channel.

In another example of the invention there is transmitted on one channela set of signals consisting alternately of speech and inverted speechand if the intervals at which switching between the two takes place areproperly chosen the result is highly unintelligible. In order to renderthis result intelligible the inverted speech must be reinverted, and inorder to do this it is necessary to switch the received signals to suchre-inverter in synchronism with the switching at the transmitter. Theinvention may be utilised to transmit the necessary synchronisingsignals and yet to prevent an unauthorised receiver making use of themto obtain intelligible signals. Thus the alternate speech and invertedspeech is transmit ted on a first channel and is also passed through adistorting network to produce signals which are mixed with thesynchronising signals transmitted on the second channel. At the receiverthe signals received on the first channel are passed through a networkidentical with that at the transmitter to produce signals which aresubtracted from those on the second channel to obtain the synchronisingsignals. These synchronising signals are then applied to control aswitch to obtain the inverted speech in the intervals of inversion whichcan be reinverted and applied in proper order with the intervals ofspeech to obtain the desired speech intelligibly.

The nature of the invention will be better understood from the followingdescription taken in conjunction with the accompanying drawing in whichFigs. 1 to, 5 are diagrams of systems according to the invention.

Figs. 6 and 7 are circuits which form part of the system of Fig. 5.

Fig. 8 is one form of network used in systems according to theinvention.

Figs. 9 and 10 are curves relating to. the network of Fig. 8, and

Figs. 11 and 12 show how a network which is the inverse of a givennetwork may be obtained.

Referring to the drawings, Fig. 1 is a diagram illustrating one systemaccording to the broadest aspect of the invention. Unintelligiblesignals are produced by mixing noise signals N with speech S to producesignals denoted N-l-S These signals are sent from a transmitting stationT on a channel I, either directl or by modulation of a carrier wave to areceiving station R. The noise signals N are transmitted from station Tto station R. on a second channel 2. At the receiver the signals N+S andN received on channels I and 2 respectively, after etection ifnecessary, are combined together in known manner to produce the originalspeech S.

In the system shown in Fig. 2, noise signals N are transmitted onchannel 2, as in the system of Fig. 1 but before being mixed with thespeech S, these signals N are passed through a network which alters themin amplitude and/or phase in a manner depending upon their frequency andthus produces other signals N/go. These latter are combined with thespeech S to roduce signals N/ +S which are transmitted on channel I Atthe receiver the signals received over channel 2 are passed through anetwork c which is an exact counterpart of that at the transmitter toproduce signals N/(p which are thereupon com- 0 bined with the signalsN/ +S received over channel I to give the original speech signals S.

In the system shown in Fig. 3 inverted speech is produced at thetransmitter but the frequency of the oscillator used is changed at thrate of a few cycles a second between limits a few hundred cycles apart.The inverted speech thus formed is denoted Iw. The signals Iw are senton channel 2, either directly or as modulations of a high frequencycarrier. The signals Iw are mixed with the varying carrier frequency Wand the mixture W-l-Iw sent over channel I either directly or asmodulations of a high frequency carrier of frequency different from thatof channel 2, or the high frequency carrier of channels I and 2 may bethe same and channel l constituted by phase or frequency modulation andchannel 2 by amplitude modulation of this carrier or vice versa.

At the receiver, the signals from channel 2 are combined with those fromchannel I, and the result, which is the varying carrier frequency,applied to a demodulator to which the varying frequency band of invertedspeech is also applied. The result is to re-invert the signals 'Iw andto cancel out the variations in frequency to yield intelligible speechS.

Fig. 4 shows the arrangement of Fig. 3 modified by the addition of adistorting network (,0. The varying frequency inverted speech istransmitted on channel 2, but before being combined with the varyingcarrier W, it is passed through a distorting network p, and the combinedsignals W+Iw/( transmitted on channel I. At the receiver the signals Iwreceived on channel 2 must be passed through a network (p identical withthat at the transmitter before the varying carrier wave W can beobtained, which latter is necessary for obtaining intelligible speechfrom the signals Iw.

The carrier frequency being outside the transmitted inverse speechfrequency range, an unauthorised listener may filter it out with aconvenient band-pass filter. It is preferable therefore to send onchannel "I, instead of the carrier frequency itself, a sub-harmonic ofthe carrier frequency within the inverse speech frequency range mixedwith the varying inverted speech. After separation from the varyinginverted speech at the receiver, in the manner described above, thissub-harmonic may then be made to yield the required variable carrierfrequency by frequency multiplication.

The arrangements shown in Figs. 2 and 4 may be modified by placing thenetwork 0 in the path of the signals N or Iw before they are transmittedover the channel 2 instead of in the position shown. At the receiver anetwork is used in the position shown for (p in Fig. 2 or 4, Which isthe inverse of the network at the transmitter. Two networks are definedto be inverse the one of the other when a signal wave passed throughboth networks in series appears in unaltered form.

Still another system according to the invention is showndiagrammatically in Fig. 5. Electronic switching means is used toconnect to a transmission channel speech signals S and speech signalsinverted I, the alternate signals transmitted being denoted Thesesignals are unintelligible unless subjected to a switching means actingin synchronism with that at the transmitter. synchronising signals forcontrolling the actuation of such switching means are denoted as s. Thesignals received in channel 2 are passed through a dis-' torting networkidentical with that at the transmitter and the synchronising signals sare obtained by subtracting the result from the signals received overchannel I. These synchronising signals are then applied to the signalsreceived over channel 2 to cause the speech to be reinverted duringintervals corresponding to those during which it was inverted at thetransmitter.

6 Fig. 6 shows an arrangement by means of which the signals of Fig. 5may be produced. The two pentode tubes V1 and V2 are connected in such amanner as to constitute a multi-vibrator. A point in the anode circuitof tube V1 is connected to the screen grid of tube V3 and acorresponding point in the anode circuit of tube V2 is connected to thescreen grid of tube V4. Speech potentials are applied between theterminals a and c in the input circuit of tube V3 and inverted speechbetween the terminals b and c in the input circuit of tube V4. Theanodes of tubes V3 and V; are connected together. The tubes V1 and V2are conducting alternately for intervals determined by the values of thecondensers C1 and C2 and the resistances R1 and R2 of the multivibratorcircuit. The potentials which appear at the point I and which areapplied to the screen grid of tube V3 may be considered as thesynchronising signals s of Fig. 5. When this point I is positive, tubeV3 is conducting and passes speech signals into the common outputcircuit. When point is negative, tube V3 is non-conducting, but point gis positive and tube V4 is conducting and passes inverted speech to thecommon output circuit. The signals appearing at point d are thus thesignals of Fig. 5.

Fig. '7 shows part of the arrangements at the receiver of Fig. 5. Afterseparation of the synchronising signals s from the signals are applied.When the synchronising signals are positive, the tube V5 is conductingand speech S appears in its output circuit. When the synchronisingsignals are negative, tube V6 is conducting and inverted speed I appearsin its out-. put circuit.

Fig. 8 shows one example of a network, which can be used as the network(p shown in Figs. 2 to 5. This is in the form of a low-pass filterterminated by a resistance R3 which is difierent from the characteristicimpedance of the filter. If this network consists of ten sections, theinductances L being 0.5 henry and the capacities C3 of 0.06 microfarad,the characteristic impedance is of the order of 1000 ohms. If thenetwork be terminated in a resistance R3 of 3,000 ohms, Fig. 9 shows therelation between the ratio of the voltages P1 and P2 at the output andinput respectively as a function of frequency, whilst Fig. 10 shows thephase change produced as a function of frequency.

If instead of constituting the network of like sections, unlike sectionsare used, still more complex curves will be obtained.

If it is desired to produce two inverse networks the following methodmay be used:

Referring to Fig. 11, let e l be the propagation constant of anamplifier and ,Be: the

7 propagation constant of the feedback path.

The propagation constant of the feedback amplifier is:

1 ldfletwfifi) Tia- 401+ Be If this circuit is considered as a network,the

inverse network must have a propagation con stant equal to:

It is assumed that a is practically constant throughout the frequencyrange and that the phase shift (p1 produced by the amplifier isproportional to frequency throughout the frequency range. This meansthat if a signal is applied to the input of the amplifier, it willappear at the output after a short time, called retardation time" andwill have the same form as the applied signal. Such amplifiers are veryfrequently used in television.

With these hypotheses,

is a constant generally less than unity and e n means that the networkhaving a propagation constant has a negative retardation time, i. e. thesignal at the output appears a short time before the signal is applied.

Consider now the circuit of Fig. 12 it comprises an artificial line ALproducing a phaseshift equal to (p in series with a network iden ticalwith the feedback path of Fig. 11.

If a signal is applied at the input l of the line, the signal willappear at the output 2 of the line after a short time and will beundistorted.

Let V1, V2, V3 he the voltages at a given frequency at terminals I, 2and 3.

Between V1 and V2 there is the relation:

. V1: z fi and between V2 and V3 there is the relation:

3 Vm n (2) valves are in parallel, on the plates we have or voltageproportional to:

i+ 3 v2[ -a+ Me] If voltage V2 is applied at the input of a networkhaving a propagation constant equal to 56: at the output we will obtaina voltage equal to On the other hand as the signal at terminals 2 hasthe same form as the signal applied at terminals I, one sees that thecircuit of Fig. 12 is the inverse network of the circuit of Fig. 11.

What is claimed is:

l. Secrecy system comprising separate first and second signal channels,a transmitting station, means at said transmitting station fortransmitting over said first channel a first set of unintelligiblesignals, means for transmitting over said second channel a second set ofdifferent unintelligi'ole signals, at least one of said sets of signalscontaining all wave compcnents from which a desired message signal isderivable, a receiving station, means at said receiving station forcombining said two sets of signals, and means for utilizing theresultant of the combined signals to derive the desired message.

2. Secrecy system comprising separate first and second signal channels,a transmitting station, means at said transmitting station for producingnoise signals consisting of a random series of frequencies continuallyvaried, means for transmitting said noise signals over said firstchannel, means at said transmitting station for combining said noisesignals with message signals and for transmitting the combined signalsover said second signal channel, a receiving station, and mean at saidreceiving station for combining the two sets of signals received oversaid two signal channels to derive the desired message.

3. Secrecy system comprising separate first and second signal channels,a transmitting station, a source of carrier frequency waves thereat,means for modulating a message signal with said carrier frequency wavesto produce a band of waves in which all the signal frequencies areinverted, means for transmitting said inverted band as a first set ofsignals over one of said signal channels, means at said transmittingstation for combining said inverted band and said carrier frequencywaves to constitute a second set of signals and for transmitting saidsecond set of signals over the other of said signal channels, areceiving station, means at said receiving station for receivingseparately said two sets of signals, means for combining said two setsof signals to derive said carrier frequency wave, and means forcombining said carrierfrequency wave with said inverted band to derivethe message signals.

4. Secrecy system according to claim 3 wherein said source of carrierfrequency waves comprises a source the frequency of which iscontinuously varied.

5. Secrecy system comprising separate first and second signal channels,a transmitting station having means for producing noise signalsconsisting of a random series of frequencies continually varied, meansfor transmitting said signals on the first channel as a first set ofsignals, a'first wave distorting network, means for passing aid noisesignals through said distorting network and combining the distortedsignals with message signals to produce a second set of signals, meansfor transmitting said second set of signals on said second channel, a,receiving station means at said receiving station for receivingseparately both said sets of signals, a second wave distorting networkidentical with that at the transmitting station, means for passing saidfirst set of signals through said second distorting network, and meansfor combining the resultant distorted signals with said second set ofsignals to yield the said message signals.

6. Secrecy system comprising separate first and second signal channels,a transmitting station, a source of carrier waves of varying frequencymeans for producing from message signals and said carrier waves a firstset of signals comprising a band of signals the frequencies of which areinverted and vary continuously with respect to those of the messagesignals, means for transmitting said first set of signals over saidfirst channel, a first wave distorting network, means for passing saidfirst set of signals through said distorting network and for combiningthe distorted band of signals with said varying frequency carrier wavesto produce a second set of signals means for transmitting the said'second set of signals over said second channel, a receiving stationmeans at said receiving station for receiving separately both said firstand second sets of signals, a second wave distorting network identicalwith that at the transmitter, means for passing said first set ofsignals through said second distorting network, and means for combiningthe said distorted signals with said second set of signals to yield saidmessage signals. 7. Secrecy system comprising separate first and secondsignal channels, a transmitting station having means for inverting aband of frequencies representing message signals, means for producingsignals consisting in successive intervals of said message signals andinverted message signals to form a first set of signals, means fortransmitting said first set of signals over said first channel, a firstWave distorting network, means for passing said signals through saiddistorting network, means for combining the resultant Waves with signalsfor synchronising said successive intervals, means for transmitting thecombined signals as a second set of signals over said second channel, areceiving station, means at said receiving station for receivingseparately said first and second sets of signals, second wave distortingnetwork identical with that at the transmitter, means for passing saidfirst set of signals through said second network, means for combiningthe wave with said second set of signals to obtain said synchronisingsignals and means for applying said synchronising signals to said firstset of signals to yield the message signals.

8. Secrecy system as claimed in claim 1 in which said separate channelsare constituted, the one by frequency or phase modulation, and the otherby amplitude modulation of the same carrier wave.

MAURICE MOISE LEVY.

